g3doc/implementation_basics.md - src.git - Git at Google

 <!-- go/cmark -->
 <!--* freshness: {owner: 'hta' reviewed: '2021-05-31'} *-->

 # Basic concepts and primitives

 ## Time

 Internally, time is represent using the [webrtc::Timestamp][1] class. This
 represents
 time with a resolution of one microsecond, using a 64-bit integer, and provides
 converters to milliseconds or seconds as needed.

 All timestamps need to be measured from the system monotonic time.

 The epoch is not specified (because we can't always know if the system clock is
 correct), but whenever an absolute epoch is needed, the Unix time
 epoch (Jan 1, 1970 at 0:00 GMT) is used.

 Conversion from/to other formats (for example milliseconds, NTP times,
 timestamp strings) should happen as close to the interface requiring that
 format as possible.

 NOTE: There are parts of the codebase that don't use Timestamp, parts of the
 codebase that use the NTP epoch, and parts of the codebase that don't use the
 monotonic clock. They need to
 be updated.

 ## Threads

 All execution happens on a TaskQueue instance. How a TaskQueue is implemented
 varies by platform, but they all have the [webrtc::TaskQueueBase][3] API.

 This API offers primitives for posting tasks, with or without delay.

 Some core parts use the [rtc::Thread][2], which is a subclass of TaskQueueBase.
 This may contain a SocketServer for processing I/O, and is used for policing
 certain calling pattern between a few core threads (the NetworkThread cannot
 do Invoke on the Worker thread, for instance).

 ## Reserved class suffixes

 C++ classes with names ending in the suffixes "Factory", "Builder" and "Manager" are supposed to behave
 in certain well known ways.

 For a particular class name Foo, the following classes, if they exist, should
 behave as follows:

 * FooFactory: Has a Create function that creates a Foo object and returns the
   object or an owning reference to it (for instance std::unique_ptr or
   rtc::scoped_refptr<Foo>). The Create function should NOT alter the factory
   state; ideally, it is marked const. Ownership of the returned object is only
   with the caller.

 * FooBuilder: Has a Build function that returns ownership of a Foo object (as
   above). The Builder can only be used once, and resources given to the Builder
   before the Build function is called are either released or owned by the Foo
   object. The Create function may be reference-qualified (declared as ```Foo
   Build() &&```), which means it is invoked as ```std::move(builder).Build()```,
   and C++ will ensure that it is not used again.

 * FooManager: Has a Create function that returns an rtc::scoped_refptr<Foo> (if
   shared ownership) or a Foo* (if the Manager retains sole ownership). If
   Create() cannot fail, consider returning a Foo&. The Manager is responsible
   for keeping track of the object; if the Create function returns a Foo*, the
   Foo object is guaranteed to be destroyed when the FooManager is destroyed.

 If a Manager class manages multiple classes of objects, the Create functions
 should be appropriately named (the FooAndBarManager would have CreateFoo() and
 CreateBar() functions), and the class will have a suitable name for the group of
 objects it is managing.

 FooFactory is mainly useful for the case where preparation for producing Foo
 objects is complex. If Foo can be created with just an argument list, consider
 exposing its constructor instead; if Foo creation can fail, consider having
 a free function called CreateFoo instead of a factory.

 Note that classes with these names exist that do not follow these conventions.
 When they are detected, they need to be marked with TODO statements and bugs
 filed on them to get them into a conformant state.

 ## Synchronization primitives

 ### PostTask and thread-guarded variables

 The preferred method for synchronization is to post tasks between threads,
 and to let each thread take care of its own variables (lock-free programming).
 All variables in
 classes intended to be used with multiple threads should therefore be
 annotated with RTC_GUARDED_BY(thread).

 For classes used with only one thread, the recommended pattern is to let
 them own a webrtc::SequenceChecker (conventionally named sequence_checker_)
 and let all variables be RTC_GUARDED_BY(sequence_checker_).

 Member variables marked const do not need to be guarded, since they never
 change. (But note that they may point to objects that can change!)

 When posting tasks with callbacks, it is the duty of the caller to check
 that the object one is calling back into still exists when the callback
 is made. A helper for this task is the [webrtc::ScopedTaskSafety][5]
 flag, which can automatically drop callbacks in this situation, and
 associated classes.

 ### Synchronization primitives to be used when needed

 When it is absolutely necessary to let one thread wait for another thread
 to do something, Thread::Invoke can be used. This function is DISCOURAGED,
 since it leads to performance issues, but is currently still widespread.

 When it is absolutely necessary to access one variable from multiple threads,
 the webrtc::Mutex can be used. Such variables MUST be marked up with
 RTC_GUARDED_BY(mutex), to allow static analysis that lessens the chance of
 deadlocks or unintended consequences.

 ### Synchronization primitives that are being removed
 The following non-exhaustive list of synchronization primitives are
 in the (slow) process of being removed from the codebase.

 * sigslot. Use [webrtc::CallbackList][4] instead, or, when there's only one
   signal consumer, a single std::function.

 * AsyncInvoker.

 * RecursiveCriticalSection. Try to use [webrtc::Mutex][6] instead, and don't recurse.

 ## Enum-To-String functions
 If there is a need to convert an enum to a string representation, such as for
 enums exposed at the Javascript API interface, the recommended way is to write
 a function named AsString, declared "static constexpr" and returning an
 absl::string_view. The declaration should be right after the enum declaration,
 in the same scope; the implementation (which must be marked "inline") should
 be at the end of the same header file.

 If the enum is not defined within a class, the "static" keyword is not needed.

 [1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/units/timestamp.h;drc=b95d90b78a3491ef8e8aa0640dd521515ec881ca;l=29
 [2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/thread.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=194
 [3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/task_queue/task_queue_base.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=25
 [4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/callback_list.h;drc=54b91412de3f579a2d5ccdead6e04cc2cc5ca3a1;l=162
 [5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/task_utils/pending_task_safety_flag.h;drc=86ee89f73e4f4799b3ebcc0b5c65837c9601fe6d;l=117
 [6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/synchronization/mutex.h;drc=0d3c09a8fe5f12dfbc9f1bcd5790fda8830624ec;l=40
	<!-- go/cmark -->
	<!--* freshness: {owner: 'hta' reviewed: '2021-05-31'} *-->

	# Basic concepts and primitives

	## Time

	Internally, time is represent using the [webrtc::Timestamp][1] class. This
	represents
	time with a resolution of one microsecond, using a 64-bit integer, and provides
	converters to milliseconds or seconds as needed.

	All timestamps need to be measured from the system monotonic time.

	The epoch is not specified (because we can't always know if the system clock is
	correct), but whenever an absolute epoch is needed, the Unix time
	epoch (Jan 1, 1970 at 0:00 GMT) is used.

	Conversion from/to other formats (for example milliseconds, NTP times,
	timestamp strings) should happen as close to the interface requiring that
	format as possible.

	NOTE: There are parts of the codebase that don't use Timestamp, parts of the
	codebase that use the NTP epoch, and parts of the codebase that don't use the
	monotonic clock. They need to
	be updated.

	## Threads

	All execution happens on a TaskQueue instance. How a TaskQueue is implemented
	varies by platform, but they all have the [webrtc::TaskQueueBase][3] API.

	This API offers primitives for posting tasks, with or without delay.

	Some core parts use the [rtc::Thread][2], which is a subclass of TaskQueueBase.
	This may contain a SocketServer for processing I/O, and is used for policing
	certain calling pattern between a few core threads (the NetworkThread cannot
	do Invoke on the Worker thread, for instance).

	## Reserved class suffixes

	C++ classes with names ending in the suffixes "Factory", "Builder" and "Manager" are supposed to behave
	in certain well known ways.

	For a particular class name Foo, the following classes, if they exist, should
	behave as follows:

	* FooFactory: Has a Create function that creates a Foo object and returns the
	object or an owning reference to it (for instance std::unique_ptr or
	rtc::scoped_refptr<Foo>). The Create function should NOT alter the factory
	state; ideally, it is marked const. Ownership of the returned object is only
	with the caller.

	* FooBuilder: Has a Build function that returns ownership of a Foo object (as
	above). The Builder can only be used once, and resources given to the Builder
	before the Build function is called are either released or owned by the Foo
	object. The Create function may be reference-qualified (declared as ```Foo
	Build() &&```), which means it is invoked as ```std::move(builder).Build()```,
	and C++ will ensure that it is not used again.

	* FooManager: Has a Create function that returns an rtc::scoped_refptr<Foo> (if
	shared ownership) or a Foo* (if the Manager retains sole ownership). If
	Create() cannot fail, consider returning a Foo&. The Manager is responsible
	for keeping track of the object; if the Create function returns a Foo*, the
	Foo object is guaranteed to be destroyed when the FooManager is destroyed.

	If a Manager class manages multiple classes of objects, the Create functions
	should be appropriately named (the FooAndBarManager would have CreateFoo() and
	CreateBar() functions), and the class will have a suitable name for the group of
	objects it is managing.

	FooFactory is mainly useful for the case where preparation for producing Foo
	objects is complex. If Foo can be created with just an argument list, consider
	exposing its constructor instead; if Foo creation can fail, consider having
	a free function called CreateFoo instead of a factory.

	Note that classes with these names exist that do not follow these conventions.
	When they are detected, they need to be marked with TODO statements and bugs
	filed on them to get them into a conformant state.

	## Synchronization primitives

	### PostTask and thread-guarded variables

	The preferred method for synchronization is to post tasks between threads,
	and to let each thread take care of its own variables (lock-free programming).
	All variables in
	classes intended to be used with multiple threads should therefore be
	annotated with RTC_GUARDED_BY(thread).

	For classes used with only one thread, the recommended pattern is to let
	them own a webrtc::SequenceChecker (conventionally named sequence_checker_)
	and let all variables be RTC_GUARDED_BY(sequence_checker_).

	Member variables marked const do not need to be guarded, since they never
	change. (But note that they may point to objects that can change!)

	When posting tasks with callbacks, it is the duty of the caller to check
	that the object one is calling back into still exists when the callback
	is made. A helper for this task is the [webrtc::ScopedTaskSafety][5]
	flag, which can automatically drop callbacks in this situation, and
	associated classes.

	### Synchronization primitives to be used when needed

	When it is absolutely necessary to let one thread wait for another thread
	to do something, Thread::Invoke can be used. This function is DISCOURAGED,
	since it leads to performance issues, but is currently still widespread.

	When it is absolutely necessary to access one variable from multiple threads,
	the webrtc::Mutex can be used. Such variables MUST be marked up with
	RTC_GUARDED_BY(mutex), to allow static analysis that lessens the chance of
	deadlocks or unintended consequences.

	### Synchronization primitives that are being removed
	The following non-exhaustive list of synchronization primitives are
	in the (slow) process of being removed from the codebase.

	* sigslot. Use [webrtc::CallbackList][4] instead, or, when there's only one
	signal consumer, a single std::function.

	* AsyncInvoker.

	* RecursiveCriticalSection. Try to use [webrtc::Mutex][6] instead, and don't recurse.

	## Enum-To-String functions
	If there is a need to convert an enum to a string representation, such as for
	enums exposed at the Javascript API interface, the recommended way is to write
	a function named AsString, declared "static constexpr" and returning an
	absl::string_view. The declaration should be right after the enum declaration,
	in the same scope; the implementation (which must be marked "inline") should
	be at the end of the same header file.

	If the enum is not defined within a class, the "static" keyword is not needed.

	[1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/units/timestamp.h;drc=b95d90b78a3491ef8e8aa0640dd521515ec881ca;l=29
	[2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/thread.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=194
	[3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/task_queue/task_queue_base.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=25
	[4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/callback_list.h;drc=54b91412de3f579a2d5ccdead6e04cc2cc5ca3a1;l=162
	[5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/task_utils/pending_task_safety_flag.h;drc=86ee89f73e4f4799b3ebcc0b5c65837c9601fe6d;l=117
	[6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/synchronization/mutex.h;drc=0d3c09a8fe5f12dfbc9f1bcd5790fda8830624ec;l=40